Heretofore, a chip fuse which is a miniature fuse has been known as one component surface-mounted on a printed circuit board of an electronic device. Such a chip fuse prevents overcurrent damage to an electronic circuit on the printed circuit board.
FIG. 12 shows a cross-sectional view of a conventional chip fuse 1. As shown in this figure, a heat-storing layer (adhesion layer) 3 made of an epoxy-based resin is formed on a surface 2a of an insulated substrate 2 that is an alumina substrate. A fuse film 4 made of copper is formed on the heat-storing layer 3. Specifically, interposing the heat-storing layer 3 between the insulated substrate 2 and the fuse film 4 prevents the fuse film 4 from coming into contact with the insulated substrate 2. Hence, when a current passes through the chip fuse 1, heat generated at a fuse element section 4b is stored in the heat-storing layer 3 without dissipating to the insulated substrate 2.
The fuse film 4 includes surface electrode sections 4a on both ends in a length direction of the chip fuse 1 (a right-left direction in FIG. 12: hereinafter, this is simply referred to as chip fuse length direction), and the fuse element section (fuse element) 4b between the surface electrode sections 4a. The fuse element section 4b is a melting section configured to be melted by heat generated at the fuse element section 4b when an overcurrent flows through the chip fuse 1. The section has a width narrower than the surface electrode sections 4a. The fuse element section 4b is provided with a plating film 5 for preventing diffusion, and a plating film 6 for facilitating the melting. The plating film 5 is a nickel film formed on the copper fuse film 4 by an electroplating method. The plating film 6 is a tin film formed on the nickel film 5 by an electroplating method.
Moreover, a first protective layer 7 made of an epoxy-based resin is formed on the fuse element section 4b (tin film 6), the first protective layer 7 serving as an undercoat. Further, a second protective layer 8 made of an epoxy-based resin is formed on the first protective layer 7, the second protective layer 8 serving as a first overcoat. A third protective layer 9 made of an epoxy-based resin is formed on the second protective layer 8, the third protective layer 9 serving as a second overcoat. A mark 10 is formed on a surface 9a of the third protective layer 9 by laser marking. The mark 10 indicates the rated current and the like of the chip fuse 1.
Backside electrodes 11 made of a silver-containing resin are formed on sections 2b-1 on both ends in the chip fuse length direction of a back surface 2b of the insulated substrate 2. End surface electrodes 12 made of a silver-containing resin are formed on end surfaces 2c on both ends in the chip fuse length direction of the insulated substrate 2. Each of the end surface electrodes 12 is formed across from the surface electrode section 4a to the backside electrode 11, electrically connecting the surface electrode section 4a and the backside electrode 11 to each other.
Moreover, the end surface electrode 12 is provided with plating films 13, 14, 15. The plating film 13 is a copper film formed on the end surface electrode 12 by an electroplating method. The plating film 14 is a nickel film formed on the copper film 13 by an electroplating method. The plating film 15 is a tin film formed on the nickel film 14 by an electroplating method. These plating films 13, 14, 15 are formed across from the surface electrode section 4a to the back surface 2b of the insulated substrate 2, covering the end surface electrode 12 and the backside electrode 11 entirely.
Incidentally, the following Patent Documents 1 to 3 are examples of prior art documents disclosing chip fuses.